What do you mean by a LaTeX 'viewer?' From the gist of the thread I feel that you are referring to a LaTeX IDE to write LaTeX documents, this wouldn't be too hard to accomplish (hell you can write LaTeX source in Notepad). If you're referring to a viewer such as Yap that reads a rendered DVI or PS file, then that's going to be much more difficult to write. I suggest Yap as a viewer, and TeXniCenter for an IDE.

There is a propiertary LaTeX IDE called 'Winedit.' It is shareware, therefore, it would not be illegal to produce and sale one yourself. I'm rather sure LaTeX is GPL'd, meaning it has a very open license. Anyone, feel free to correct me here.